Every time he sensed her gaze on him he could feel her, begging for a reason why he didn't love her, and he couldn't think of one. He could think of plenty of reasons why he could love her, and therein lay the problem. Because wasn't that sort of the point of love? To shake the tiresome yoke of rationality in favour of something altogether more essential?
The Doctor was a clever man. He could have taught Einstein a thing or two about the way that light travels, and held his own in a debate with Lenin. Damn it, he even helped Shakespeare out. But for the life of him, if someone asked him to state one good reason why he'd loved that little London shopgirl so much, he'd have been flummoxed. And that, for him, was what made the whole love caboodle so utterly brilliant. His feelings weren't some equation, that could be proved without room for argument, yet they held the same certainty. It was a bit like leaping headlong into a black hole, being in love with someone (which was ironic, because he'd fallen even deeper in love with someone whilst preparing to leap into a black hole. Happy days.)
And because he could see logically why one might fall in love with Martha Jones, he utterly failed to do so himself. He could clinically observe the sensuous sway of her hips, and reason that she was, by definition, sexy. But that very act of defining her attraction, metaphorically putting his finger on it, left him utterly devoid of any attraction to her. Yet Rose… Rose he could watch sleep for hours, enjoying the sensation that he hadn't a clue why the curve of her neck, the sweep of her eyelashes, was so utterly absorbing.
He simply could not talk himself into being in love with Martha, even had he especially wanted to, even if there had been room in his hearts. Because his feelings weren't dictated by his own internal diplomacy. Because when there was reasoning there could not be love, and when there was love there could not be reason.
After all, people didn't say 'madly' in love for nothing.